As systems, such as the multimedia entertainment, communications and diagnostic systems utilized by the automotive and aerospace industries, become more complex, a need arises for additional devices to communicate, either with each other or with a central controller or the like. Historically, these systems included dedicated wiring extending between the various devices in order to support communications therebetween. As systems have become more integrated and the communications requirements have been increased, the amount of dedicated wiring that would be required can quickly become excessively large, both in terms of the space required for the wiring and the cost of the wiring and the attendant installation. Moreover, as the amount of dedicated wiring increases, the overall complexity of the system also generally increased as well as the likelihood that some portion of the wiring might be damaged or broken during or following installation.
As such, network buses have been developed to provide a common communication path between a plurality of devices. In automotive and aerospace applications, for example, a network bus can be utilized to monitor various components and to collect diagnostic and status information. In this regard, diagnostic and status information relating to the strain, acceleration, pressure and/or temperature to which the various components are subjected may be collected and analyzed. By way of further example, a network bus architecture is currently being developed to support communications and the delivery of multimedia information to the occupants of a vehicle, such as an automobile, minivan, sports utility vehicle, aircraft, boat or the like. Advantageously, this network bus would transport the audio signals, including streaming audio signals, produced by one or more of a radio, a cassette tape player, a compact disc player or the like to selected speakers or headphone jacks throughout the vehicle. Similarly, the network bus may support voice and data communications with a cellular telephone carried by an occupant of the vehicle, as well as communications with a laptop computer, a handheld computing device or the like. Also, the network bus may transmit video signals, including streaming video signals, from a television receiver, a videocassette recorder or other video source to one or more video monitors. In addition, the network bus may transmit sensor and actuator signals to and from devices such as drivetrain devices, passive restraint devices, crash avoidance devices, drive-by-wire devices, or the like.
In addition to the variety of devices that are connected to a network bus, one or more controllers are also generally connected to the network bus for receiving data from the various devices and for sending commands to the devices. Among other things, these commands specify the manner in which the various devices are to function including the manner in which the various devices are to transmit information over the network bus. Additionally, the controller(s) can receive input from an operator, such as an occupant of the vehicle. This input can include, for example, an indication of the source(s) of the signals to be transmitted over the network bus as well as the destination of the signals.
Traditionally, networks of the type described above have transmitted data in analog format. Unfortunately, analog signals are susceptible to noise introduced into the signals during data transmission. Given that many of the transmitted signals have a low amplitude to start with, this noise can corrupt the signal and decrease the signal to noise ratio to levels that cause loss of resolution in the signal. Further, as many of these network devices are scattered some distance from the controller, the electrical lines connecting the network devices to the controller may be sufficiently long to cause signal degradation due to DC resistance in the wiring.
In light of these shortcomings, it would be advantageous to utilize digital networks. But, many conventional digital networks suffer from a variety of problems themselves. For example, many existing digital networks operate according to complicated protocols which require each network device to have a relatively high level processor, thereby increasing the cost of the network devices. Complicated protocols also introduce overhead into the messages on the bus that are not necessary for data acquisition and control. This overhead can severely limit the number of data samples that can be transmitted on the bus. These networks also have other problems. For example, some do not support both acquisition and control. Further, these networks typically have bulky network device interfaces, slow network communication rates and/or a low network device count. Additionally, many computer systems that include digital networks do not operate in a time-deterministic manner. As such, these computer systems generally lack the capability to schedule a trigger command to the network components that repeats or is interpreted and executed with any precision timing.
Regardless of the digital or analog nature of network, the network bus may be damaged during or following installation. In this regard, the network bus typically consists of a plurality of conductors or wires that may extend great lengths between the various controllers and network devices. Due to accidents or other unforeseen circumstances, one or more of the wires may be broken, thereby creating an open circuit. Thus, components on one side of the open circuit will be unable to communicate via the broken conductor with components on the other side of the open circuit. Additionally, signals transmitted over the broken conductor will be reflected by the broken end of the conductor due to the impedance mismatch. The reflected signals will then be returned along the conductor, thereby interfering, both constructively and destructively, with other signals being transmitted via the conductor. While the components on one side of the open circuit may be able to communicate at relatively low communication rates, such as ten kilobits per second, reflected signals will generally prevent effective communications between the components at higher communication rates such as ten megabits per second or the like. Moreover, the open circuit will render the broken conductor more susceptible to noise, thereby further limiting effective communications.
In instances in which one or more conductors of the network bus are broken, one of two different approaches has generally been taken. According to one approach, the network bus remains unrepaired for at least some period of time rendering the network bus useless until repairs are made. The network may utilize a redundant bus, which, after the damaged network bus is retired, carries the communications for the network until the damaged network bus is repaired. In addition, communications can continue over the network bus, albeit at a relatively slow communication rate that is selected so as not to be corrupted by the reflected signals. Since a number of applications require that communications be conducted via the network bus at relatively high communication rates, the intentional slowing of the data over the network bus to reduce, if not negate, the deleterious impact of reflected signals may be inappropriate. Alternatively, communications via the network bus can be halted and the technician can troubleshoot the network bus to identify the break in the network bus and can then physically repair the broken conductors. Once the repairs have been completed, the communications over the network bus can be recommenced. However, the physical repair of the network bus oftentimes requires that the network bus be removed from service for some period of time, which action may also be inappropriate for certain applications, such as time-sensitive applications, safety-critical applications, applications that result in some type of liability if inoperable, or other applications that demand continuous monitoring or feedback.
Accordingly, it would be advantageous to develop an improved network bus that could accommodate bus failure caused by an open circuit condition, such as the breaking of one or more conductors of the network bus. Moreover, it would be desirable for the improved network bus to support continued communications between the devices connected to the network bus after the open circuit condition, without having to remove the network bus from service in order to physically repair the network bus.
In addition to problems with open circuits in a network, there may also be problems associated with provision of excessive current to a device on the network. To protect the often complex and costly remote devices of many networks, a circuit breaker or fuse is connected at a distribution area to a power supply that supplies input current to the remote devices, such as via the network bus. Typically, circuit breakers and fuses interrupt the current in an electric circuit, sometimes referred to as tripping the circuit breaker or fuse, when the current through the circuit becomes higher than that allowed by the circuit breaker or fuse. Conventional circuit breakers and fuses are typically rated for a specific current level that generally depends upon the components in the circuit and their current tolerances. When the current through the circuit breaker exceeds the rated current level, the circuit breaker or fuse trips and interrupts the current in the circuit.
In one type of conventional circuit breaker, such as a mechanical circuit breaker, when enough current runs through the circuit to trip the circuit breaker, a pair of contacts that are normally in contact in order to conduct current through the circuit breaker and the rest of the circuit are separated, such as by preloaded springs, thus breaking the circuit. Generally, a conventional fuse includes a strip of a low-melting metal that is connected in series with the circuit. Because of its electrical resistance, the metal strip is heated by current through the circuit. And if the current exceeds the current rating of the fuse, the strip melts, thus breaking the circuit.
While conventional circuit breakers and fuses are used in many power systems, they pose some problems. Generally, conventional circuit breakers and fuses are positioned at the distribution point between the power source and the network bus to protect the network bus and all of the devices as a whole instead of the devices individually. But because different remote devices can have different current tolerances, a single conventional circuit breaker or fuse cannot optimally protect all of the remote devices of the network from excessive current.
Occasionally in networks employing a single conventional circuit breaker or fuse the remote devices can draw an excessive amount current that is over the current tolerance of the respective remote devices, such as during defective operation of one or more remote devices. In such an instance, it would be desirable for the circuit breaker or fuse to break the circuit to protect the remote devices drawing the excessive amount of current. But in some instances the excessive amount of current is below the current rating of the circuit breaker or fuse and, therefore, the circuit breaker or fuse provides no protection for the remote devices drawing the excessive amount of current. In such an instance, the affected remote devices suffer from an unnecessary danger of damage caused by overheating and even fire due to the excessive amount of current. Conversely, if the current rating of the circuit breaker or fuse is set according to the remote device with the lowest current tolerance, the circuit breaker or fuse will trip if remote devices having higher current tolerances attempt to draw current levels within their tolerance but below the lowest current tolerance. As shown, a single circuit breaker or fuse connected to the network bus at a distribution point is undesirable because remote devices typically have different current tolerances. Because a single circuit breaker or fuse results in all-or-nothing power to the remote devices on the network since, if one remote device requires protection from excessive current, all of the devices on the network must receive the same protection, even if it is unnecessary.
In addition to the inability of a single conventional circuit breaker or fuse to optimally protect all of the remote devices of the network from excessive current, conventional mechanical circuit breakers and fuses suffer from limitations due to their material characteristics. Most conventional circuit breakers and fuses cannot be adjusted for different requirements without replacing the entire circuit breaker. For example, if a mechanical circuit breaker is rated for a ten amp trip and is attached to a circuit containing a remote device rated for five amps, the ten amp rated circuit breaker would need to be replaced with a five amp rated circuit breaker to provide over-current protection for the five amp rated remote device.
One method by which to protect all of the remote devices from individual excessive current situations is to provide an individually current rated circuit breaker or fuse to each remote device connected to the network bus, in addition to the circuit breaker or fuse connected at the distribution point to protect the network bus. But disadvantageously, such a method leads to an increase in the cost, size and weight of the network because it requires added components (i.e., circuit breakers). Additionally, because such a method merely employs multiple conventional circuit breakers, it suffers from the same material drawbacks as conventional circuit breakers and fuses in that it does not allow for adjustments to account for changes in the characteristics of the remote device, such as replacing the remote device with one current rating with a remote device with a different current rating.
Accordingly, it would be advantageous to develop an improved system to control the input current to the remote devices to protect remote devices on an individualized basis based upon the respective current tolerances of each remote device without adding additional circuit breakers to the network. Moreover, it would be desirable for the improved system to allow for adjustments in the current rating protection afforded to the remote devices to account for changes in the current tolerance of the remote device.